Is the engine knock discussed elsewhere harmful?

I've read many of the threads here about the occasional knocking sound that happens on startup. One question that hasn't been explicitly asked is "is it actually harmful?". It's interesting that there are a lot of reports of knocking, but no reports about any resulting engine damage.

Another question: does anybody know or have any theories as to what pieces are banging together to actually make the noise? I imagine it's not the connecting rods, or else there would be lots of engines with bad bearings by now. My own theory is that each time the engine misfires its going to cause an unexpected change in RPM and torque that is felt all the way from the crank to MG1. Maybe we're hearing the protests of the poor torque damper caught in the middle.

This is what happens when I have an hour+ commute to think about that awful noise I just heard starting the car

 

That is an excellent video and I had seen it before. It does imply that the problem is that a cylinder isn't firing when the noise occurs (because it's just sucked in a puddle from the manifold?), but that doesn't tell us what's actually making the noise.

If I wanted to be really scientific about it, I'd reproduce the noise as much as possible until the engine failed and then report back on what broke. My car is still under warranty so it would get fixed on Toyota's dime. Unfortunately I'm not that brave

 

I have not, for several reasons:
1) It doesn't happen frequently, and after driving to the dealership there would be no reliable way to reproduce the problem.
2) Based on other's experience on PriusChat, there is nothing the dealer could do to remedy the problem. Note that my Prius is a 2014 model, and should already have all the changes that are available.
3) It doesn't really bother me that much. I'm asking more out of technical curiosity than anything.

 

Yes, and I alluded to those posts as "discussed elsewhere" right in the subject of this post. I'm really looking to take the discussion beyond those other discussions. Other than the noise, are there any consequences to this problem? It sounds like a piston is about to come through the hood, but has that actually happened to anybody? Are there any documented cases of actual engine failure associated with the noise?

 

Ah, that would explain why he quoted Prius Team and not me. Ever since our election down here I'm going with the first thought that pops into my head and running with it.

 

I hadn't seen those threads and still haven't. However based on your post I was eventaully able to search the internet for "prius gear train spark plug" and found http://www.aecs.net/techniek2012/March%20prius.pdf which says

Why the knock

What was causing the terrible knock? The hybrid drive train works with very precise position sensors. The management system needs to anticipate where the rotor of the electric motor is going to be every 65353’st part of a third of a rotation. This so it can power up each of the coils in the stator with the appropriate amount of current; - do not make the magnetic field too strong as you get over shoot of the rotor and also - do not give it too little.
The engine is connected to the rotors of the MG’s via the planetary gear set.
Any misfire in the engine gives an MG rotor undershoot, so the MG ECU is going to compensate for this incorrect rotor position by increasing current to the next MG coil. This will be aided by the next cylinder firing, causing overshoot, which needs to be compensated for…. Please do not think that when the engine is running the MG works just as a generator. The MG’s both do use current, also when charging, as this is how the MG ECU controls the charge voltage to the battery.

There is in any drive train free play, even in the big ends on the crankshaft. Move all the free play of every item in one direction and immediately in the opposite direction, and presto there is your noise!​

Which is exactly the information I was looking for.

 

While I wouldn't say it's good for any of the parts involved, I wouldn't expect it to be catastrophically damaging either. Gears generally don't slide against each other, so unless the forces are high enough to fatigue the metal it shouldn't cause any wear. It's sort of like clapping your hands, you get a lot of noise but you don't abrade your skin.